Deposition of fluorosilsesquioxane films

ABSTRACT

There is provided an array of fluoro-substituted silsesquioxane thin film precursors having a structure wherein fluoro groups are bonded to the silicon atoms of a silsesquioxane cage. In a first aspect, the present invention provides a composition comprising a vaporized material having the formula [F—SiO 1.5 ] x [H—SiO 1.5 ] y , wherein x+y=n, n is an integer between 2 and 30, x is an integer between 1 and n and y is a whole number between 0 and n. Also provided are films made from these precursors and objects comprising these films.

This application is a divisional of allowed application Ser. No.09/420,052, filed Oct. 18, 1999, now U.S. Pat. No. 6,440,550.

BACKGROUND OF THE INVENTION

Semiconductors are widely used in integrated circuits for electronicapplications, including high-speed computers and wirelesscommunications. Such integrated circuits typically use multipletransistors fabricated in single crystal silicon. Many integratedcircuits now contain multiple levels of metallization forinterconnections. A single semiconductor microchip may have thousands,and even millions of transistors. Logically, a single microchip may alsohave millions of lines interconnecting the transistors. As devicegeometries shrink and functional density increases, it becomesimperative to reduce the capacitance between the lines. Line-to-linecapacitance can build up to a point where delay time and crosstalkhinders device performance. Reducing the capacitance within thesemulti-level metallization systems reduces the RC constant, crosstalkvoltage, and power dissipation between the lines. Typically, thin filmsof silicon dioxide are used as dielectric layers and to reduce thecapacitance between functional components of the device.

Such dielectric thin films serve many purposes, including preventingunwanted shorting of neighboring conductors or conducting levels, byacting as a rigid, insulating spacer; preventing corrosion or oxidationof metal conductors, by acting as a barrier to moisture and mobile ions;filling deep, narrow gaps between closely spaced conductors; andplanarizing uneven circuit topography so that a level of conductors canthen be reliably deposited on a film surface which is relatively flat. Asignificant limitation is that typically interlevel dielectric (ILD) andprotective overcoat (PO) films must be formed at relatively lowtemperatures to avoid destruction of underlying conductors. Another veryimportant consideration is that such dielectric films should have a lowrelative dielectric constant k, as compared to silicon dioxide (k=3.9),to lower power consumption, crosstalk, and signal delay for closelyspaced conductors.

Recently, attempts have been made to use materials other than silicondioxide. Notable materials include low-density materials, such asaerogels and silsesquioxanes. The dielectric constant of a porousdielectric, such as a silicon dioxide aerogel, can be as low as 1.2.This lower dielectric constant results in a reduction in the RC delaytime. However, methods of making aerogels require a supercritical dryingstep. This step increases the cost and the complexity of semiconductormanufacturing.

Films deposited from hydrogen silsesquioxane (HSQ) resins have beenfound to possess many of the properties desirable for ILD and POapplications. For example, Haluska et al. (U.S. Pat. No. 4,756,977, Jul.12, 1988) describe a film deposition technique comprising diluting in asolvent a hydrogen silsesquioxane resin, applying this as a coating to asubstrate, evaporating the solvent and ceramifying the coating byheating the substrate in air. Others have found that by ceramifying sucha coating in the presence of hydrogen gas (Ballance et al., U.S. Pat.No. 5,320,868, Jun. 14, 1994) or inert gas (European Patent Application90311008.8), the dielectric constant of the final film may be loweredand/or stabilized as compared to ceramifying in air. Each of thesepatents discloses the use of silsesquioxane resin dissolved in asolvent. The resulting silsesquioxane solution is coated onto asubstrate by a spin-on coating technique. Although these coatings formuseful dielectric layers after curing, as device sizes progressivelyminimize, it is necessary to have available dielectric thin films havinga lower dielectric constant than that provided by the simple hydrogensilsesquioxane films.

Films comprising fluorinated siloxane components have low dielectricconstants and are appropriate for integrated circuit applications. Thefluorinated films are generally formed by CVD of a silicon-containingcomponent and a small molecule that provides a fluorine source.Generally, this process requires the use of fluorine gas, which is bothtoxic and corrosive. Other methods utilize compounds that are both asource of carbon and fluorine. For example, Lee et al., U.S. Pat. No.5,660,895, Aug. 26, 1997, discloses a process for the low temperatureplasma-enhanced CVD of silicon oxide films using disilane as a siliconprecursor and carbon tetrafluoride as a fluorine source.

Although limited effort has been directed towards chemical vapordeposition of silsesquioxane dielectric coatings. See, Gentle, U.S. Pat.No. 5,279,661, Jan. 18, 1994 disclosing CVD of hydrogen silsesquioxanecoatings on a substrate, efforts to construct thin films fromfluorinated silsesquioxanes have not been reported.

An array of low k thin films of different composition and precursors forthese films which can be deposited onto a substrate using CVD wouldrepresent a significant advance in the art and would open avenues forcontinued device miniaturization. Quite surprisingly, the presentinvention provides such films and precursors.

SUMMARY OF THE INVENTION

It has now been discovered that silsesquioxanes having fluoro groupsbonded to the silicon atoms of the silsesquioxane cage are usefulprecursors for low dielectric constant thin films. The fluorinatedsilsesquioxane cages are easily prepared using art-recognized techniquesand volatile fractions of these molecules can be deposited ontosubstrates using CVD. Following its deposition onto a substrate, thefluorinated silsesquioxane layer is cured, producing a low k dielectriclayer or film.

In a first aspect, the present invention provides a compositioncomprising a material having the formula[F—SiO_(1.5)]_(x)[H—SiO_(1.5)]_(y), wherein x+y=n, n is an integerbetween 2 and 30, x is an integer between 1 and n and y is a wholenumber between 0 and n.

In a second aspect, the present invention provides a method of forming alow k dielectric film. The method comprises vaporizing and depositing ona substrate a material having the formula[F—SiO_(1.5)]_(x)[H—SiO_(1.5)]_(y), wherein x+y=n, n is an integerbetween 2 and 30, x is an integer between 1 and n and y is a wholenumber between 0 and n.

In a third aspect, the invention provides a low k dielectric filmcomprising a material having the formula[H_(a)SiO_(b)]_(c)[F_(a)SiO_(b)]_(d). In this formula, a is less than orequal to 1, b is greater than or equal to 1.5 and c and d are membersindependently selected so that they are both greater than 10.

In a fourth aspect, the present invention provides an object comprisinga low k dielectric film comprising a material having the formula[H_(a)SiO_(b)]_(c)[F_(a)SiO_(b)]_(d). In this formula, a is less than orequal to 1, b is greater than or equal to 1.5, and c and d areindependently selected and they are both greater than 10.

These and other aspects and advantages of the present invention will beapparent from the detailed description that follows.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a group of three-dimensional structural formulae for thefluoro-substituted silsesquioxanes of the invention.

DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENTS

Abbreviations and Definitions

-   -   “CVD,” as used herein, refers to “chemical vapor deposition.”    -   “FHSQ,” as used herein refers to “fluorinated hydrogen        silsesquioxanes.”    -   “FSQ,” as used herein, refers to “fluorinated silsesquioxanes.”    -   “FSX,” as used herein, refers to “fluorinated siloxanes.”    -   “FHSX,” as used herein, refers to “fluorinated hydrogen        siloxanes.”

The terms “fluorinated silsesquioxanes” and “fluorinated hydrogensilsesquioxanes” are used herein to describe various silane compoundshaving the formula [F—SiO_(1.5)]_(x)[H—SiO_(1.5)]_(y), wherein x+y=n, nis an integer between 2 and 30, x is an integer between 1 and n and y isa whole number between 0 and n. “Fluorinated silsesquioxanes” refer tosilsesquioxanes in which substantially every silicon atom has a fluorogroup attached thereto. “Fluorinated hydrogen silsesquioxanes” refer tosilsesquioxanes having a mixture of fluorinated silicon atoms andsilicon atoms bearing hydrogen. “Silsesquioxane” is used genericallyherein to refer to both of the above-described species.

Though not explicitly represented by this structure, these resins maycontain a small number of silicon atoms which have either 0 or 2hydrogen atoms or fluoro groups attached thereto due to various factorsinvolved in their formation or handling.

The terms “fluorinated siloxane film” and “fluorinated hydrogen siloxanefilm” refer to films resulting from curing the deposited silsesquioxane.The films have the generic formula [H_(a)SiO_(b)]_(c)[F_(a)SiO_(b)]_(d),wherein a is less than or equal to 1, b is greater than or equal to 1.5and c and d are independently selected and they are both greater than10.

“Low k,” as used herein, refers to a dielectric constant that is lowerthan that of a SiO₂ film.

Introduction

The present invention is based on the discovery that volatile fractionsof fully fluorinated silsesquioxanes and fluorinated hydrogensilsesquioxanes can be used to form coatings on various substrates. Thevolatile compounds are deposited onto a substrate, such as asemiconductor wafer by CVD. Following their deposition, the film iscured to produce a low k dielectric film. The films produced by thetechniques described herein are valuable as protective and dielectriclayers on substrates such as electronic devices.

The invention provides methods of forming low k dielectric films.Additionally, there is provided an array of low k films and compoundsuseful for forming these films.

The Compounds

In a first aspect, the present invention provides a compositioncomprising a vaporized material having the formula[F—SiO_(1.5)]_(x)[H—SiO_(1.5)]_(y), wherein x+y=n, n is an integerbetween 2 and 30, x is an integer between 1 and n and y is a wholenumber between 0 and n. Presently preferred % fluorine content is fromabout 2% to about 75%, more preferably from about 10% to about 50%.

The silsesquioxanes of the invention are fluoro-substituted moleculesthat, at higher values of n, exist as cage “T-n” molecules (e.g., T-8,T-10, etc.). In a preferred embodiment, n is an integer with a valuefrom 6 to 16. In a further preferred embodiment, n is an integer with avalue from 8 to 12.

These compounds can be synthesized by a number of art-recognizedmethods. For example FSQ can be synthesized by the hydrolysis andcondensation of F—Si—X₃.

In the formula provided above, X represents a species that is eliminatedduring hydrolysis. Currently, preferred X groups are halogens, alkoxygroups and aryloxy groups, more preferably halogens and even morepreferably Cl.

The hydrolysis/condensation reactions preferably result in a fullycondensed FSQ or FHSQ or the hydrolysis and/or condensation may beinterrupted at an intermediate point such that partial hydrolysates(containing Si—OR, Si—Cl, etc.) and/or partial condensates (containingSiOH groups) are formed. See, for example, Olsson, Arkiv. Kemi. 13:367-78(1958); Barry et al., J. Am. Chem. Soc. 77: 4248-52(1955); andDittmar et al., J. Organomet. Chem. 489:185-194(1995). In a preferredembodiment, the reaction produces substantially fully condensedsilsesquioxanes. The fluorine content of the ASQ and AHSQ can becontrolled by manipulation of the stoichiometry of the hydrolysisreaction.

The hydrolysis/condensation reactions can be performed in a number ofdifferent reaction milieus and the choice of appropriate reactionconditions is well within the abilities of those of skill in the art.The hydrolysis and condensation polymerization is generally carried outusing conventional equipment, by the addition of the fluorosilanemonomer (or both monomers in the case of copolymerization) to an aqueousmedium. The aqueous medium can be simply water or it can be an aqueousalcohol. Additionally, catalysts such as organic and/or inorganic acidsor bases can be added to the reaction mixture. For example, when silanealkoxides are utilized as precursors for the silsesquioxane, it is oftendesirable to use an acidic catalyst (e.g., HCl) to facilitate thereaction. Moreover, when silane halides are utilized as precursors, abasic reaction environment often facilitates the reaction. See, forexample, Wacker et al., U.S. Pat. No. 5,047,492, Sep. 10, 1991.

The silane monomers (e.g., FSiCl₃, FSi(OEt)₃, HSiCl₃, HSi(OEt)₃, etc.)can be added neat to the hydrolysis mixture or they can be firstsolubilized in a solvent (e.g., hexane, etc.). The monomer(s) ispreferably added at a measured rate to the hydrolysis medium to obtainmore precise control of the hydrolysis and condensation. In a preferredembodiment, wherein two or more monomers are utilized, a mixture of themonomers is formed and then this mixture is added to the hydrolysismixture.

Additional control of the hydrolysis and condensation polymerizationreactions can also be obtained though adjustment of the temperature ofthe hydrolysis reaction medium, by maintaining the reaction temperaturein the range of about 0° C. to about 50° C. Preferably, the temperatureof the hydrolysis reaction medium is maintained at a temperature fromabout 0° C. to about 5° C.

Certain of the starting hydrogen silsesquioxane used in the fluorinationreaction are commercially available. For example, the T-8 cage iscommercially available (Aldrich Chemical Co., Dow Corning, Hitachi).Moreover, various methods for the production of hydrogen silsesquioxaneshave been developed. For instance, Collins et al. in U.S. Pat. No.3,615,272, which is incorporated herein by reference, describe a processof forming nearly fully condensed hydrogen silsesquioxane (which maycontain up to 100-300 ppm silanol) comprising hydrolyzingtrichlorosilane in a benzenesulfonic acid hydrate hydrolysis medium andthen washing the resulting product with water or aqueous sulfuric acid.Similarly, Bank et al. in U.S. Pat. No. 5,010,159, Apr. 23, 1991,disclose methods of forming hydrogen silsesquioxanes comprisinghydrolyzing hydridosilanes in an arylsulfonic acid hydrate hydrolysismedium to form a resin which is then contacted with a neutralizingagent. A preferred embodiment of this latter process uses an acid tosilane ratio of about 6/1.

Higher order silsesquioxane cages (e.g., T-10, -12, etc.) can beprepared by, for example, partial rearrangement of octasitsesquioxanecages and fluorinated octasilsesquioxane cages. These rearrangementreactions are catalyzed by compounds such as sodium acetate, sodiumcyanate, sodium sulfite, sodium hydroxide and potassium carbonate. Thereactions are generally carried out in an organic solvent, preferablyacetone. See, for example, Rikowski et al., Polyhedron 16:3357-3361(1997).

Recovery of the silsesquioxane reaction product from the aqueousreaction medium may be carried out using conventional techniques (e.g.,solvent extraction with organic solvents that solubilize the reactionproduct but are immiscible with the aqueous reaction medium),salting-out of the silsesquioxane reaction product, and the like. Thesilsesquioxane reaction product can then be recovered by filtration orevaporation of the extract solvent as applicable.

The compounds can be purified by techniques common in the art of organicchemistry including chromatography (e.g., gel permeation, silica gel,reverse-phase, HPLC, FPLC, etc.), crystallization, precipitation,fractionation, ultrafiltration, dialysis and the like. In a presentlypreferred embodiment, the reaction material is purified by fractionationand precipitation.

Art-recognized analytical methods can be used to characterize thecompounds. Useful methods include spectroscopic techniques (e.g., ¹H,¹³C, ¹⁹F NMR, infrared), mass spectrometry, gel permeationchromatography against a molecular weight standard, elemental analysis,melting point determination and the like. In a preferred embodiment, thecompounds are characterized by a protocol involving each of thesetechniques.

In an exemplary embodiment, fluorotrichlorosilane (˜25 g) is addeddrop-wise with stirring to distilled water (˜250 mL) and a non-polarsolvent (e.g., hexanes, toluene) at a temperature of about 0° C. Uponcompletion of the addition of the silane, the reaction is allowed tostir for between 10 minutes and 24 hours. If a precipitate forms in theaqueous mixture, this mixture can be clarified by filtration orcentrifugation. An organic solvent such as hexane is then added to theaqueous reaction medium. The resulting mixture is stirred for a timesufficient to allow the extraction of the reaction product from theaqueous medium. The organic layer is removed from the aqueous layer andthe aqueous phase is extracted further with three washings of theorganic solvent (˜3×100 mL). The hexane washings are combined with theoriginal organic phase extract, and the combined organic phase solutionis dried by contacting it with sodium sulfate, and thereafter it isfiltered. After evaporation of the solvent from the organic phaseextract, the recovered reaction product is dried under high vacuum toyield the desired product. The product can be characterized by ¹H and²⁹Si NMR, mass spectrometry and elemental analysis. The molecular weightof the product is determined by GPC relative to a standard, such as apolystyrene calibration standard.

Low k dielectric films with desirable physical properties can also beprepared using copolymers of fluorosilanes copolymerized withtrichlorosilane. In an exemplary embodiment, an FHSQ monomer is preparedby the hydrolysis condensation method. In this embodiment, the fluorocontent of the final product is controlled by the stoichiometric ratioof the fluorosilane to trichlorosilane. Thus, to prepare a product thathas an average of 3:1 fluorine to hydrogen, fluorotrichlorosilane (3moles) and trichlorosilane (1 mole) are combined, and the combinedcomponents are added dropwise with stirring to distilled water (˜250 mL)and a non-polar solvent at a temperature of about 0° C. After stirringfor a period of from about 10 minutes to about 24 hours, an organicsolvent such as hexane (˜250 mL) is added to the aqueous reaction mediumto extract the reaction product from the aqueous medium, and thereaction mixture is stirred for ten minutes. The work up of the reactionmixture and the characterization of the product are substantiallysimilar to that described for the homopolymer above.

In a preferred embodiment, the FSQ or FHSQ polymer is deposited by vapordeposition, however, certain of the above-described reactions can leadto the production of FSQ and FHSQ polymers that have a molecular weightthat is too high to allow these polymers to be vaporized in usefulquantities. Although, the volatile fraction can be vaporized and used toform a film, leaving behind the higher molecular weight fraction, in apreferred embodiment, the high molecular weight molecules are removedfrom the more volatile components of the product mixture prior to usingthese compounds for vapor deposition.

Separation of the high and low molecular weight fractions can beaccomplished by a number of means including, for example, gel permeationchromatography, high performance liquid chromatography (HPLC),ultrafiltration, fractional crystallization and solvent fractionation.Each of these methods is well known in the art and it is within theabilities of one of skill in the art to devise an appropriatepurification protocol for a particular mixture without undueexperimentation. When other deposition methods are utilized, thevolatility of the film precursor is less of a concern.

In a preferred embodiment, using a vapor deposition method, the productmixture is fractionated to obtain the low molecular weight species thatcan be volatilized in the deposition process of this invention. Anyconventional technique for fractionating the polymer can be used herein.Particularly preferred, however, is the use of a variety of fluids at,near or above their critical point. This process is described inHanneman et al., U.S. Pat. No. 5,118,530, Jun. 2, 1992. The processdescribed therein comprises (1) contacting the H-resin with a fluid at,near or above its critical point for a time sufficient to dissolve afraction of the polymer; (2) separating the fluid containing thefraction from the residual polymer; and (3) recovering the desiredfraction.

Specifically, the fractionation method involves charging an extractionvessel with a silsesquioxane product mixture and then passing anextraction fluid through the vessel. The extraction fluid and itssolubility characteristics are controlled so that only the desiredmolecular weight fractions of silsesquioxane are dissolved. The solutionwith the desired fractions of silsesquioxane is then removed from thevessel leaving those silsesquioxane fractions not soluble in the fluidas well as any other insoluble materials such as gels or contaminants.The desired silsesquioxane fraction is then recovered from the solutionby altering the solubility characteristics of the solvent and, thereby,precipitating out the desired fraction. These precipitates can then becollected by a process such as filtration or centrifugation.

The extraction fluid used in this process includes any compound which,when at, near or above its critical point, will dissolve the fraction ofsilsesquioxane desired and not dissolve the remaining fractions.Additional consideration, however, is usually given to the criticaltemperature and pressure of the solvent compound so that unreasonablemeasures are not necessary to reach the appropriate point. Examples ofspecific compounds that are functional include, but are not limited to,carbon dioxide and most low molecular weight hydrocarbons such as ethaneor propane. Additional methods of fractionation are disclosed inKatsutoshi et al., U.S. Pat. No. 5,486,546, Jan. 23, 1996.

By such methods, one can recover the desired fraction of an FHSQ or FSQ.Other equivalent methods, however, which result in obtaining thefractions described herein are also contemplated. For instance, methodssuch as solution fractionation or sublimation function herein (See, forexample, Olsson et al., Arkiv. Kemi 13: 367-78(1958)).

The preferred fraction of silsesquioxane used in the process of thisinvention is one that can be volatilized under moderate temperatureand/or vacuum conditions. Generally, such fractions are those in whichat least about 75% of the species have a molecular weight less thanabout 3000. Preferred herein, however, are those fractions in which atleast about 75% of the species have a molecular weight less than about1800, with those fractions in which at least about 75% of the specieshave a molecular weight between about 800 and 1600 being particularlypreferred.

In preferred embodiments, this molecular weight range will correspond tocompounds that are T-2 to T-30 cages. For vapor deposition, preferredspecies correspond to compounds that are T-2 to T-16, and for spin onapplications, T-12 to T-30.

Additionally, it is contemplated that mixtures of silsesquioxanescontaining components that are not easily vaporized can be used hereinas the source of silsesquioxane vapor. Volatilization of such mixtures,however, can leave a residue comprising nonvolatile species. Thisresidue does not constitute an impediment to the use of silsesquioxanemixtures containing compounds having abroad range of molecular weights.

Chemical Vapor Deposition (CVD)

Any deposition method known in the art can be used to produce a filmusing one or more compounds of the invention. Deposition techniques ofgeneral applicability include, for example, spraying (e.g., nebulizerunder vacuum), spin-on, dip-coating, sputtering, CVD, and the like.Other coating methods will be apparent to those of skill in the art.

As the use of CVD is presently preferred, in a second aspect, theinvention provides a method of forming a low k dielectric film. Themethod comprises vaporizing and depositing on a substrate a materialhaving the formula [F—SiO_(1.5)]_(x)[H—SiO_(1.5)]_(y), wherein x+y=n, nis an integer between 2 and 30, x is an integer between 1 and n and y isa whole number between 0 and n.

In an exemplary embodiment, the desired fraction of silsesquioxane isobtained, and it is placed into a CVD apparatus, vaporized andintroduced into a deposition chamber containing the substrate to becoated. Vaporization can be accomplished by heating the silsesquioxanesample above its vaporization point, by the use of vacuum, or by acombination of the above. Generally, vaporization is accomplished attemperatures in the range of 50° C.-300° C. under atmospheric pressureor at lower temperature (near room temperature) under vacuum.

The amount of silsesquioxane vapor used in the process of this inventionis that which is sufficient to deposit the desired coating. This canvary over a wide range depending on factors such as the desired coatingthickness, the area to be coated, etc. In addition, the vapor may beused at nearly any concentration desired. If dilute vapor is to be used,it may be combined with nearly any compatible gas such as air, argon orhelium.

The process of this invention can be used to deposit desirable coatingsin a wide variety of thicknesses. For instance, coatings in the range offrom about a monolayer to greater than about 2-3 microns are possible.Greater film thicknesses are possible where end use applications warrantsuch thicker films. Multiple coating applications of layered thin filmsare preferred for preparing ceramic films that are 4 microns or more inthickness, to minimize stress cracking.

These coatings may also cover, or be covered by other coatings such asSiO₂ coatings, SiO₂/modifying ceramic oxide layers, silicon containingcoatings, silicon carbon containing coatings, silicon nitrogencontaining coatings, silicon nitrogen carbon containing coatings,silicon oxygen nitrogen containing coatings, and/or diamond like carboncoatings. Such coatings and their mechanism of deposition are known inthe art. For example, many are taught in Haluska, U.S. Pat. No.4,973,526, Nov. 27, 1990.

The formation of the films of the invention is accomplished by a largevariety of techniques, which can conceptually be divided into twogroups: (1) film growth by interaction of a vapor-deposited species withthe substrate; and (2) film formation by deposition without causingchanges to the substrate or film material. See, for example, Bunshah etal., DEPOSITION TECHNOLOGIES FOR FILMS AND COATINGS, Noyes, Park Ridge,N.J., 1983; and Vossen et al., THIN FILM PROCESSES, Academic Press,N.Y., N.Y., 1978.

The second group is most relevant to the present invention and itincludes another three subclasses of deposition: (a) chemical vapordeposition, or CVD, in which solid films are formed on a substrate bythe chemical reaction of vapor phase chemicals that contain the requiredconstituents; (b) physical vapor deposition, or PVD, in which thespecies of the thin film are physically dislodged from a source to forma vapor which is transported across a reduced pressure region to thesubstrate, where it condenses to form the thin film; and (c) coating ofthe substrate with a liquid, which is then dried to form the solid thinfilm. When a CVD process is used to form single-crystal thin films, theprocess is termed epitaxy. The formation of thin films by PVD includesthe processes of sputtering and evaporation.

There are currently three major types of chemical vapor deposition (CVD)processes, atmospheric pressure CVD (APCVD), low pressure (LPCVD) andplasma enhanced CVD (PECVD). Each of these methods has advantages anddisadvantages. The choice of an appropriate CVD method and device for aparticular application is well within the abilities of those of skill inthe art.

Atmospheric pressure CVD (APCVD) devices operate in a mass transportlimited reaction mode at temperatures of approximately 400° C. Inmass-transport limited deposition, temperature control of the depositionchamber is less critical than in other methods: mass transport processesare only weakly dependent on temperature. As the arrival rate of thereactants is directly proportional to their concentration in the bulkgas, maintaining a homogeneous concentration of reactants in the bulkgas adjacent to the wafers is critical. Thus, to insure films of uniformthickness across a wafer, reactors that are operated in the masstransport limited regime must be designed so that all wafer surfaces aresupplied with an equal flux of reactant. The most widely used APCVDreactor designs provide a uniform supply of reactants by horizontallypositioning the wafers and moving them under a gas stream.

In contrast to APCVD reactors, low pressure CVD (LPCVD) reactors operatein a reaction rate-limited mode. In processes that are run underreaction rate-limited conditions, the temperature of the process is animportant parameter. To maintain a uniform deposition rate throughout areactor, the reactor temperature must be homogeneous throughout thereactor and at all wafer surfaces. Under reaction rate-limitedconditions the rate at which the deposited species arrive at the surfaceis not as critical as constant temperature. Thus, LPCVD reactors do nothave to be designed to supply an invariant flux of reactants to alllocations of a wafer surface.

Under the low pressure of an LPCVD reactor, for example, operating atmedium vacuum (30-250 Pa or 0.25-2.0 torr) and higher temperatures(550-600° C.), the diffusivity of the deposited species is increased bya factor of approximately 1000 over the diffusivity at atmosphericpressure. The increased diffusivity is partially offset by the fact thatthe distance across which the reactants must diffuse increases by lessthan the square root of the pressure. The net effect is that there ismore than an order of magnitude increase in the transport of reactantsto the substrate surface and by-products away from the substratesurface.

LPCVD reactors are designed in two primary configurations: (a)horizontal tube reactors; and (b) vertical flow isothermal reactors.Horizontal tube, hot wall reactors are the most widely used LPCVDreactors in VLSI processing. They are employed for depositing poly-Si,silicon nitride, and undoped and doped SiO₂ films. They find such broadapplicability primarily because of their superior economy, throughput,uniformity, and ability to accommodate large diameter (e.g., 150 mm)wafers.

The vertical flow isothermal LPCVD reactor further extends thedistributed gas feed technique, so that each wafer receives an identicalsupply of fresh reactants. Wafers are again stacked side by side, butare placed in perforated-quartz cages. The cages are positioned beneathlong, perforated, quartz reaction-gas injector tubes, one tube for eachreactant gas. Gas flows vertically from the injector tubes, through thecage perforations, past the wafers, parallel to the wafer surface andinto exhaust slots below the cage. The size, number, and location ofcage perforations are used to control the flow of reactant gases to thewafer surfaces. By properly optimizing cage perforation design, eachwafer can be supplied with identical quantities of fresh reactants fromthe vertically adjacent injector tubes. Thus, this design can avoid thewafer-to-wafer reactant depletion effects of the end-feed tube reactors,requires no temperature ramping, produces highly uniform depositions,and reportedly achieves low particulate contamination.

The third major CVD deposition method is plasma enhanced CVD (PECVD).This method is categorized not only by pressure regime, but also by itsmethod of energy input. Rather than relying solely on thermal energy toinitiate and sustain chemical reactions, PECVD uses an rf-induced glowdischarge to transfer energy into the reactant gases, allowing thesubstrate to remain at a lower temperature than in APCVD or LPCVDprocesses. Lower substrate temperature is the major advantage of PECVD,providing film deposition on substrates not having sufficient thermalstability to accept coating by other methods. PECVD can also enhancedeposition rates over those achieved using thermal reactions. Moreover,PECVD can produce films having unique compositions and properties.Desirable properties such as good adhesion, low pinhole density, goodstep coverage, adequate electrical properties, and compatibility withfine-line pattern transfer processes, have led to application of thesefilms in VLSI.

PECVD requires control and optimization of several depositionparameters, including rf power density, frequency, and duty cycle. Thedeposition process is dependent in a complex and interdependent way onthese parameters, as well as on the usual parameters of gas composition,flow rates, temperature, and pressure. Furthermore, as with LPCVD, thePECVD method is surface reaction limited, and adequate substratetemperature control is thus necessary to ensure uniform film thickness.

CVD systems usually contain the following components: (a) gas sources;(b) gas feed lines; (c) mass-flow controllers for metering the gasesinto the system; (d) a reaction chamber or reactor; (e) a method forheating the wafers onto which the film is to be deposited, and in sometypes of systems, for adding additional energy by other means; and (f)temperature sensors. LPCVD and PECVD systems also contain pumps forestablishing the reduced pressure and exhausting the gases from thechamber.

In a preferred embodiment, the films of the invention are produced usingCVD with a heated substrate.

Curing

In a third aspect, the invention provides a low k dielectric filmcomprising a material having the formula[H_(a)SiO_(b)]_(c)[F_(a)SiO_(b)]_(d). In this formula, a is less than orequal to 1, b is greater than or equal to 1.5, and c and d areindependently selected and are both greater than 10, more preferably100, more preferably 1000, more preferably 10,000 and more preferably100,000.

In a preferred embodiment, the precursor for this film is asilsesquioxane having the formula [F—SiO_(1.5)]_(x)[H—SiO_(1.5)]_(y),wherein x+y=n, n is an integer between 2 and 30, x is an integer between1 and n, and y is a whole number between 0 and n. This film is formed bycuring the film of silsesquioxane monomer deposited onto the substrate.

Prior to initiating the curing process, a film reflow process can beperformed to smooth the surface of the film. After coating,silsesquioxane film reflow can be effected by raising the temperature ofthe substrate to a temperature between 120° C. and 200° C., typicallyfor about 5 minutes. This step may be done in air, or in the curingambient, at a convenient pressure (typically atmospheric). Alternately,this step can be combined with the following curing step under mostcuring conditions applicable to an ILD or PO deposition.

Prior art silsesquioxane-derived films have been cured in variousambients, resulting in widely varying properties. These ambients includeair, ammonia, nitrogen, nitrogen/argon, and hydrogen/nitrogen.Generally, temperatures of about 400° C. and curing times of about 30minutes to an hour are also taught in the prior art. In particular, ithas been found that curing in air produces a predominantly Si—O film,curing in ammonia produces a silicon oxynitride type film, and curing ininert or reducing atmospheres results in films which retain some portionof the Si—H bonding inherent in uncured hydrogen silsesquioxane.

The present invention is comprehended for use in silsesquioxane filmsdried and cured in all ambients, including reducing or inert ambientsother than those discussed herein. Even films that are carefully curedunder non-oxidizing conditions may eventually become exposed to moistureand/or oxygen, either during further processing of the device, duringpackaging, or in use. The invention is also comprehended for use withdeposition methods that use trace amounts of a group VIII catalyst, suchas Pt(acac)₂, to further silsesquioxane film curing.

The formation of a silsesquioxane thin film is effected by processingthe coated substrate, via treatment at moderately elevated temperaturesor with UV irradiation, or an incident electron beam to convert thesilsesquioxane molecule composition into a silsesquioxane thin film.This crosslinking conversion is carried out in a moisture-containingatmosphere containing at least about 0.5% relative humidity andpreferably containing from about 15% relative humidity to about 100%relative humidity. The specified level of moisture may be present in theatmosphere during the entire processing procedure for forming theceramic thin film or, alternatively, can be present during only aportion of the procedure.

In addition to the moisture-containing atmosphere, and inert gases suchas nitrogen, argon, helium or the like may be present or reactive gasessuch as air, oxygen, hydrogen chloride, ammonia and the like may bepresent.

In one embodiment of this invention, the conversion of thesilsesquioxane molecule on the coated substrate is accomplished viathermal processing, by heating the coated substrate. The temperatureemployed during the heating to form the thin film is moderate,preferably being at least about 100° C., more preferably at least about150° C. Extremely high temperatures, which are often deleterious toother materials present on the substrate, e.g., particularly metallizedelectronic substrates, are generally unnecessary. Heating temperaturesin the range of about 150° C. to about 700° C. are preferable, withtemperatures in the range of about 200° C. to about 500° C. being morepreferred. The exact temperature will depend on factors such as theparticular substituted organosilsesquioxane molecule utilized, thecomposition of the atmosphere (including relative humidity), heatingtime, coating thickness and coating composition components. Theselection of appropriate conditions is well within the abilities ofthose of skill in the art.

Heating is generally conducted for a time sufficient to form the desiredthin film. The heating period typically is in the range of up to about 6hours. Heating times of less than about 2 hours, e.g., about 0.1 toabout 2 hours, are preferred.

The heating procedure is generally conducted at ambient pressure (i.e.,atmospheric pressure), but subatmospheric pressure or a partial vacuumor superatmospheric pressures may also be employed. Any method ofheating, such as the use of a convection oven, rapid thermal processing,hot plate, or radiant or microwave energy is generally functional. Therate of heating, moreover, is also not critical, but it is mostpractical and preferred to heat as rapidly as possible.

In an alternative embodiment of this invention, the formation of asilsesquioxane thin film is accomplished by subjecting the coatedsubstrate to ultraviolet (UV) irradiation or an electron beam. Exposureof the coated substrate to such irradiation has been found to effect thedesired crosslinking conversion of the silsesquioxane molecule in thecoated substrate. The irradiation treatment is ordinarily carried outwithout subjecting the coated substrate to the elevated temperaturesused in the thermal processing, but combinations of the irradiation andthermal processing treatments could be employed, if desired.

The silsesquioxane thin films formed using irradiation-based processingare generally characterized as having higher SiO₂ contents thantypically result from thermal processing under otherwise identicalcoating conditions. An advantage of the use of irradiation-basedprocessing is that patterned films may be generated on a substrate bythe selective focusing of the radiation.

Characterization

Although the properties of a bulk material are well characterized, thesame material in its thin film form can have properties that aresubstantially different from those of the bulk material. One reason isthat thin film properties are strongly influenced by surface properties,while in bulk materials this is not the case. The thin film, by its verydefinition, has a substantially higher surface-to-volume ratio than doesa bulk material. The structure of thin films, and their method ofpreparation also play a vital role in determining the film properties.

There exists an array of art-recognized techniques for characterizingthin films, including specular and off-specular x-ray and neutronreflectivity, energy-dispersive x-ray reflectivity, total externalreflectance x-ray fluorescence MeV ion scattering atomic forcemicroscopy and ellipsometry. See, for example, Lin et al., Proc. ACSPMSE 77: 626(1997); Wolf et al., SILICON PROCESSING FOR THE VLSI ERA,Volume 1 (Process Technology) (Lattice Press, Sunset Beach, Calif.1986), incorporated herein by reference.

Film thickness can be determined using commercially availableinstruments such as a Nanospec AFT. Correction of film thickness forrefractive index is frequently desirable. Refractive index of thin filmscan be measured using an elipsometer. Such devices are commerciallyavailable (Rudolph). Other methods exist to characterize surfaceroughness, film integrity, dielectric constant and the like. Thesemethods are briefly described below. It is well within the abilities ofone of skill in the art to choose an appropriate method for determininga desired characteristic of a film of the invention.

The out-of-plane thermal expansion of the thin films can be measuredusing a capacitance cell. The sample is used to measure the capacitanceof a precision parallel-plate capacitor of constant area so that themeasured capacitance is inversely proportional to the actual samplethickness. These measurements are typically made under conditions ofcontrolled humidity.

The surface roughness of films occurs as a result of the randomness ofthe deposition process. Real films almost always show surface roughness,even though this represents a higher energy state than that of aperfectly flat film. Depositions at high temperatures tend to show lesssurface roughness. This is because increased surface mobility from thehigher substrate temperatures can lead to filling of the peaks andvalleys. On the other hand, higher temperatures can also lead to thedevelopment of crystal facets, which may continue to grow in favoreddirections, leading to increased surface roughness. At low temperatures,the surface roughness as measured by surface area, tends to increasewith increased film thickness. Oblique deposition that results inshadowing, also increases surface roughness. Epitaxial and amorphousdeposits have shown measured surface area nearly equal to thegeometrical area, implying the existence of very flat films. This hasbeen confirmed by Scanning Electron Micrography (SEM) examination ofthese films. Thus, in a preferred embodiment, the surface roughness ofthe films of the invention is investigated by SEM and/or AFM. In apreferred embodiment, thin films of the invention are characterizedfurther by being uniform and crack-free when viewed by electronmicrography.

Infrared spectroscopy is also useful to characterize the films of theinvention. For example, FTIR spectroscopy can provide informationregarding the structure of films formed at different cure temperatures.Different cure temperatures will frequently produce films havingdifferent IR spectra. Moreover, infrared spectroscopy can be used todetermine the silanol content of the thin film.

The organization of the film into a crystalline or amorphous structurecan be determined using X-ray diffraction.

The density of the films of the invention can be varied by selection ofthe film precursors. Porosity develops during crosslinking ofsilsesquioxane molecules during the curing stage. The porosity ofcondensed silsesquioxane films is known to be a function of curetemperature: both thickness and porosity decrease with increasing curetemperature due to densification. The density of a thin film providesinformation about its physical structure. Density is preferablydetermined by weighing the film and measuring its volume. If a film isporous from the deposition process, it generally has a lower densitythan the bulk material.

The dielectric constant of a particular film can be measured by theMOSCAP method which is known to one of skill in the art. When the filmis a component of a device with interconnect lines, the line-to-linecapacitance measurements can be carried out, for example, by using a0.50/0.50 μm width/spacing comb structure. Other methods for measuringdielectric constants can be applied to the films of the presentinvention.

Substrate

The choice of substrates to be coated is limited only by the need forthermal and chemical stability at the temperature and in the environmentof the deposition vessel. Thus, the substrate can be, for example,glass, metal, plastic, ceramic or the like. It is particularly preferredherein, however, to coat electronic devices to provide a protective ordielectric coating.

In an exemplary embodiment, the substrate is a semiconductor substrate(e.g., of silicon). The substrate is functionalized with conductorswhich may be, for instance, formed of an aluminum-0.5% copper alloy. Thedielectric films of the present invention need not be deposited directlyover a conducting layer (i.e., other dielectric layers may intervene ora conducting layer may not be present below the dielectric film of thepresent invention). In general, the dielectric film is deposited by,e.g., CVD of a silsesquioxane film precursor over the substrate,followed by reflow and film curing steps, which may be combined toconvert the film to a final form. Either during reflow or curing (orbetween these steps), the film is typically subjected to a temperaturebetween 120° C. and 200° C. for a period of time sufficient to producesilsesquioxane reflow and enhance the planarization of film.

In those substrates having interconnect lines, a metal layer isdeposited and etched to form interconnect lines. Any number ofinterconnect lines and interconnect line geometries can be present.Interconnect lines typically have a vertical thickness on the order of0.5-2.0 micron and a horizontal thickness which varies by design, butwill typically be in the range of 0.25 to 1 micron. After the formationof interconnect, a thin layer of a film of the invention or another film(e.g., silicon dioxide) with a thickness on the order of 0.2-5.0 microncan optionally be deposited over the surface of the structure.

Objects Incorporating the Films

In another aspect, the invention provides an object comprising a low kdielectric film, said film comprising a material having the formula[H_(a)SiO_(b)]_(c)[F_(a)SiO_(b)]_(d). In this formula a is less than orequal to 1; b is greater than or equal to 1.5; and c and d are membersindependently selected from the group consisting of the integers greaterthan 10. Although the films of the invention can be incorporated intoessentially any device or object in which a low k dielectric film wouldhave utility, in a preferred embodiment, the object comprises a wafer,preferably made of a material acting as a semiconductor.

Semiconductor wafers made of a variety of materials are well known inthe art and substantially all of these wafers are appropriate forcoating with the films of the invention. In a preferred embodiment, thecomprises a member selected from Si, SiON, SiN, SiO₂, Cu, Ta, TaN andcombinations thereof, more preferably Si, SiO₂ and combinations thereof.

In another preferred embodiment, the wafer is metallized, preferablywith a member selected from copper, titanium, titanium nitride andcombinations thereof.

It is understood that the examples and embodiments described herein arefor illustrative purposes only and that various modifications or changesin light thereof will be suggested to persons skilled in the art and areto included within the spirit and purview of this application and areconsidered within the scope of the appended claims. All publications,patents, and patent applications cited herein are hereby incorporated byreference in their entirety for all purposes.

1. A method of forming a porous low k dielectric film, said methodcomprising: providing a vaporizable material having the formula:[F—SiO_(1.5)]_(x)[H—SiO_(1.5)]_(y), wherein: x+y=n; n is an integerbetween 2 and 30; x is an integer between 1 and n; and y is a numberbetween 0 and n; vaporizing the vaporizable material to form a vaporizedmaterial; depositing the vaporized material onto a substrate; and curingthe vaporized material.
 2. The method of claim 1, wherein n is a memberselected from the group consisting of the integers from 6 to 16,inclusive.
 3. The method of claim 2, wherein n is a member selected fromthe group consisting of the integers from 8 to
 12. 4. The method ofclaim 1, wherein about 75% of the vaporizable material has a molecularweight of less than about 3000 daltons.
 5. The method of claim 4,wherein about 75% of the vaporizable material has a molecular weight ofless than about 1800 daltons.
 6. The method of claim 5, wherein about75% of said vaporizable material has a molecular weight of less thanabout 1600 daltons.
 7. The method of claim 1, wherein depositingcomprises vapor deposition, sputtering or combinations thereof.
 8. Themethod of claim 7, wherein vapor deposition comprises chemical vapordeposition, physical vapor deposition or combinations thereof.
 9. Themethod of claim 8, wherein chemical vapor deposition comprisesatmospheric chemical vapor deposition, low pressure chemical vapordeposition, plasma enhanced chemical vapor deposition or combinationsthereof.
 10. The method of claim 1, wherein the vaporizable materialcomprises a film precursor.
 11. The method of claim 1, whereinvaporizing is carried out at a temperature of from about 500° C. toabout 300° C.
 12. The method of claim 1, wherein vaporizing is performedunder vacuum.
 13. The method of claim 1, wherein vaporizing is performedunder vacuum.
 14. The method of claim 1, wherein curing comprisesultraviolet light, electron beam or combinations thereof.
 15. The methodof claim 14, wherein curing is accomplished by heating to a temperatureof from about 150° C. to about 700° C.
 16. The method of claim 15,wherein said temperature is from about 200° C. to about 500° C.
 17. Themethod of claim 1, wherein the vaporizable material comprises a CVDprecursor.
 18. A method of forming a porous low k dielectric film, saidmethod comprising: providing a vaporizable material having the formula:[F—SiO_(1.5)]_(x)[H—SiO_(1.5)]_(y), wherein: x+y=n; n is an integerbetween 2 and 30; x is an integer between 1 and n; and y is a numberbetween 0 and n; vaporizing the vaporizable material to form a vaporizedmaterial; depositing the vaporized material onto a substrate; andsubjecting the vaporized material to radiation from a radiation source.19. The method of claim 18, wherein n is a member selected from thegroup consisting of the integers from 6 to 16, inclusive.
 20. The methodof claim 19, wherein n is a member selected from the group consisting ofthe integers from 8 to
 12. 21. The method of claim 18, wherein about 75%of the vaporizable material has a molecular weight of less than about3000 Daltons.
 22. The method of claim 21, wherein about 75% of thevaporizable material has a molecular weight of less than about 1800Daltons.
 23. The method of claim 22, wherein about 75% of saidvaporizable material has a molecular weight of less than about 1600Daltons.
 24. The method of claim 18, wherein depositing comprises vapordeposition, sputtering or combinations thereof.
 25. The method of claim24, wherein vapor deposition comprises chemical vapor deposition,physical vapor deposition or combinations thereof.
 26. The method ofclaim 25, wherein chemical vapor deposition comprises atmosphericchemical vapor deposition, low pressure chemical vapor deposition,plasma enhanced chemical vapor deposition or combinations thereof. 27.The method of claim 18, wherein the vaporizable material comprises afilm precursor.
 28. The method of claim 18, wherein vaporizing iscarried out at a temperature of from about 50° C. to about 300° C. 29.The method of claim 28, wherein vaporizing is performed under vacuum.30. The method of claim 18, wherein vaporizing is performed undervacuum.
 31. The method of claim 18, wherein subjecting to radiation froma radiation source comprises ultraviolet radiation, electron beamradiation or combinations thereof.
 32. The method of claim 18, whereinthe vaporizable material comprises a CVD precursor.